RBL Checking

The Mail Abuse Protection System’s Real-time Blackhole List
(MAPS RBL) is a list of host and networks that are known to be friendly or
neutral to abusers who use these hosts and networks to either originate or
relay spam, or to provide spam support services.

You can configure your MTAs to compare incoming connections against
the MAPS RBL. You can also use DNS-based databases used to determine incoming
SMTP connections that might send unsolicited bulk mail.